The present invention relates to a switching mode power supply. It is known that switching mode power supplies of all types are widely used to supply power to electronic equipment requiring stabilized voltages.
The switching mode power supplies receive an input an AC main voltage and through rectifying circuits, filters and switching devices, transformers, output rectifying bridges and filters, generate an output of one or more DC supply voltages, stabilized with great precision.
However, the generation of a regulated direct voltage is not the sole function carried out by the power supplies. They must also provide for some forms of protection in order to interrupt operation in case of short circuits, overloads and overcurrent.
It is also required that they be started gradually (soft-start) and that, in general, the electrical isolation from the mains and the output supply circuits be guaranteed.
Because of the complexity of the power supply requirements many and differing embodiments have been proposed. In order to ensure the electrical isolation between the mains and the output circuits a transformer is used, however this does not suffice.
Current flowing through the transformer primary winding is under the control of switching devices, generally one or more transistors, which must receive driving signals from the regulating circuits. These, in turn, in order to carry out the regulation function, must receive signals coming from different locations.
Clearly if the regulation function is to control the output voltage it must receive a feedback signal from the output circuits.
At the same time, however, since it must protect the power supply from overloads, it should receive a signal proportional to the current drained by the primary voltage, since it is not very convenient to monitor the overloads as the sum of the various loads on the different output circuits.
Finally, for the regulating circuits to carry out their function, they must be provided with a supplied voltage.
Two basic approaches have been derived from these contrasting requirements and they have been adopted up to now for the design and production of power supplies.
One of these approaches is to connect and supply the regulating circuits with power directly obtained from the mains by means of auxiliary power supplies. The output voltage signal present at the output circuits of the main power supply is transmitted to the regulating circuits through relatively expensive decoupling devices such as optoelectronic couplers or current switching devices combined with transformers.
In the second approach, although the regulating circuits are connected to the output circuits they are supplied with power from the mains by auxiliary power supplies which require the use of a transformer for the electrical isolation. The control signals which drive the current switching devices at the primary side are supplied by transformers which guarantee the electrical isolation. Also, the primary current feedback signal used by the regulating circuits to prevent overloads is transferred to the regulating circuits through a current transformer. This results in a wide use of transformers which make the embodiment relatively expensive and cumbersome.
U.S. Pat. Nos. 4,128,867 and 4,092,711 are exemplary of this second approach, whereas, an example of the first approach is shown in the publication "Application Note" U-90, page 948, published by UNITRODE CORPORATION, with reference to the use of a control integrated circuit available under code UC1524A.
Integrated circuits are now available which carry out the various regulation and control functions as referred to, with the sole addition of few external circuit elements.
As explained above, they can be used with both the mentioned approaches, with all the related constraints and drawbacks which have been indicated.